The present invention relates to silicon aluminum oxynitride-boron nitride composites and articles and to a method of making the composites and articles, and more particularly, to a multiphase ceramic composite of silicon aluminum oxynitride and boron nitride, a window transparent to electromagnetic radiation made therefrom and to a method for making the same.
High-speed vehicles, such as high-speed aircraft, may be required to pass through a hostile environment. Under these conditions, it may become necessary to provide protection for certain communication and detection apparatus carried by such a vehicle from sources of heat shock and pressure shock. In such high-speed vehicles, such shock is typically due to frictional contact with the ambient environment. Protection from heat and pressure shock must be provided without detracting from the ability of the protected apparatus to communicate with the outside of the vehicle by means of electromagnetic radiation within the frequency spectrum of interest. A common way of providing the desired protection is to place the apparatus behind a suitable window, which is transparent to the desired frequency spectrum of electromagnetic radiation, sometimes referred to as an electromagnetic window or an antenna window.
Currently available compositions for providing such an electromagnetic window or antenna window often lack the requisite mechanical strength, toughness and thermal shock resistance. In U.S. Pat. No. 4,666,873, Morris, Jr. and Tanzilli have provided an improved window transparent to electromagnetic radiation in a predetermined frequency spectrum wherein the window is made from a ceramic material containing about 0.01 percent to about 35 percent by volume boron nitride and aluminum nitride. In U.S. Pat. No. 4,666,873, which is incorporated herein by reference in its entirety, an article is fabricated by mixing powders of the specified materials; shaping the mixture into the desired form of the article; and densifying the mixture in a non-oxidizing medium at a temperature, pressure and time sufficient to form a structural ceramic. Morris, Jr., and Tanzilli found that the resulting ceramic article could be used to protect apparatus from damage due to heat or pressure shock without detracting from the ability of the apparatus to communicate through the ceramic article (window) by means of electromagnetic radiation in a predetermined frequency spectrum.
Although U.S. Pat. No. 4,666,873 provides a ceramic article, such as an electromagnetic window, having excellent characteristics, it is always desirable to provide alternative composites and articles made therefrom wherein one or more of the properties are improved. A composite for use as an electromagnetic window ideally possesses transparency to electromagnetic radiation in a predetermined frequency spectrum over a wide temperature range, congruent vaporization kinetics, appropriate dielectric properties, thermal shock resistance over a wide range of heating rates, mechanical strength and toughness, spall resistance, high refractoriness, and a low erosion rate and ablation rate.